Hydroprocessing includes processes which convert hydrocarbons in the presence of hydroprocessing catalyst and hydrogen to more valuable products. Hydrocracking is a hydroprocessing process in which hydrocarbons crack in the presence of hydrogen and hydrocracking catalyst to lower molecular weight hydrocarbons. Depending on the desired output, a hydrocracking reactor may contain one or more fixed beds of the same or different catalyst.
In hydrocracking, feeds contain concentrations of polycyclic aromatic and aliphatic rings which have low cetane value. Polycyclic ring molecules or compounds are organic molecules that are composed of alkylated forms of multiple aromatic or aliphatic rings or combinations thereof. The alkylated multiple rings can be fused such as in a naphthalene or can be alkylated with a degree of branching, or connected to other single or multiple fused rings via one or more alkyl groups. The alkylated polycyclic rings can also include aliphatic rings with either partially saturated single rings or fused rings like alkylated tetralins or fully saturated rings like the alkylated decalins. The smallest polycyclic ring compounds are bicyclic ring compounds which may comprise fused rings or two rings connected by an alkyl group and each of which rings may be aromatic or aliphatic.
It is desirable to open the rings of these polycyclic compounds having more than two rings to reduce them to bicyclic compounds such as naphthalenes and naphthenes and open the rings of the bicyclics to crack them into alkyl naphthenes and paraffins. Ring opening typically requires aromatic rings to be saturated before the ring can be opened. While opening the rings of the bicyclic compounds, it is desirable to preserve all of the original carbon atoms on the original bicyclic molecule rather than truncating the bicyclic molecule to smaller paraffins, aromatics and cycloalkanes. The alkyl naphthenes and paraffins that retain all of the original carbon atoms on the original bicyclic molecule contribute to a higher cetane number in the recovered diesel product stream. The smaller paraffins, aromatics and cycloalkanes end up in the naphtha boiling range thereby diminishing the resulting diesel selectivity.
Two-stage hydrocracking processes involve fractionation of a hydrocracked stream from a first stage hydrocracking reactor followed by hydrocracking of an unconverted oil (UCO) stream in a second stage hydrocracking reactor. However, the best two-stage hydrocracking process cannot achieve full conversion to materials boiling below the diesel cut point. Typically, a bottoms stream from the fractionation column in two-stage hydrocracking unit comprises a UCO stream that is recycled to the second stage hydrocracking reactor for further conversion in a sweet environment. UCO is concentrated with bicyclic aromatic and aliphatic compounds that are desirably cracked into compounds boiling in the diesel range.
Better catalyst compositions are desired to open polycyclic aromatic and aliphatic rings while preserving more of the original carbon atoms on the molecule during hydrocracking.